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Subject[tip:x86/mm] mm, x86/mm: Fix performance regression in get_user_pages_fast()
Commit-ID:  5b65c4677a57a1d4414212f9995aa0e46a21ff80
Gitweb: http://git.kernel.org/tip/5b65c4677a57a1d4414212f9995aa0e46a21ff80
Author: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
AuthorDate: Sat, 9 Sep 2017 00:56:03 +0300
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 13 Sep 2017 11:26:52 +0200

mm, x86/mm: Fix performance regression in get_user_pages_fast()

The 0-day test bot found a performance regression that was tracked down to
switching x86 to the generic get_user_pages_fast() implementation:

http://lkml.kernel.org/r/20170710024020.GA26389@yexl-desktop

The regression was caused by the fact that we now use local_irq_save() +
local_irq_restore() in get_user_pages_fast() to disable interrupts.
In x86 implementation local_irq_disable() + local_irq_enable() was used.

The fix is to make get_user_pages_fast() use local_irq_disable(),
leaving local_irq_save() for __get_user_pages_fast() that can be called
with interrupts disabled.

Numbers for pinning a gigabyte of memory, one page a time, 20 repeats:

Before: Average: 14.91 ms, stddev: 0.45 ms
After: Average: 10.76 ms, stddev: 0.18 ms

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Thorsten Leemhuis <regressions@leemhuis.info>
Cc: linux-mm@kvack.org
Fixes: e585513b76f7 ("x86/mm/gup: Switch GUP to the generic get_user_page_fast() implementation")
Link: http://lkml.kernel.org/r/20170908215603.9189-3-kirill.shutemov@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
mm/gup.c | 97 ++++++++++++++++++++++++++++++++++++++--------------------------
1 file changed, 58 insertions(+), 39 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 23f01c4..4a789f1 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1618,6 +1618,47 @@ static int gup_p4d_range(pgd_t pgd, unsigned long addr, unsigned long end,
return 1;
}

+static void gup_pgd_range(unsigned long addr, unsigned long end,
+ int write, struct page **pages, int *nr)
+{
+ unsigned long next;
+ pgd_t *pgdp;
+
+ pgdp = pgd_offset(current->mm, addr);
+ do {
+ pgd_t pgd = READ_ONCE(*pgdp);
+
+ next = pgd_addr_end(addr, end);
+ if (pgd_none(pgd))
+ return;
+ if (unlikely(pgd_huge(pgd))) {
+ if (!gup_huge_pgd(pgd, pgdp, addr, next, write,
+ pages, nr))
+ return;
+ } else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {
+ if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,
+ PGDIR_SHIFT, next, write, pages, nr))
+ return;
+ } else if (!gup_p4d_range(pgd, addr, next, write, pages, nr))
+ return;
+ } while (pgdp++, addr = next, addr != end);
+}
+
+#ifndef gup_fast_permitted
+/*
+ * Check if it's allowed to use __get_user_pages_fast() for the range, or
+ * we need to fall back to the slow version:
+ */
+bool gup_fast_permitted(unsigned long start, int nr_pages, int write)
+{
+ unsigned long len, end;
+
+ len = (unsigned long) nr_pages << PAGE_SHIFT;
+ end = start + len;
+ return end >= start;
+}
+#endif
+
/*
* Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
* the regular GUP. It will only return non-negative values.
@@ -1625,10 +1666,8 @@ static int gup_p4d_range(pgd_t pgd, unsigned long addr, unsigned long end,
int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
struct page **pages)
{
- struct mm_struct *mm = current->mm;
unsigned long addr, len, end;
- unsigned long next, flags;
- pgd_t *pgdp;
+ unsigned long flags;
int nr = 0;

start &= PAGE_MASK;
@@ -1652,45 +1691,15 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
* block IPIs that come from THPs splitting.
*/

- local_irq_save(flags);
- pgdp = pgd_offset(mm, addr);
- do {
- pgd_t pgd = READ_ONCE(*pgdp);
-
- next = pgd_addr_end(addr, end);
- if (pgd_none(pgd))
- break;
- if (unlikely(pgd_huge(pgd))) {
- if (!gup_huge_pgd(pgd, pgdp, addr, next, write,
- pages, &nr))
- break;
- } else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {
- if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,
- PGDIR_SHIFT, next, write, pages, &nr))
- break;
- } else if (!gup_p4d_range(pgd, addr, next, write, pages, &nr))
- break;
- } while (pgdp++, addr = next, addr != end);
- local_irq_restore(flags);
+ if (gup_fast_permitted(start, nr_pages, write)) {
+ local_irq_save(flags);
+ gup_pgd_range(addr, end, write, pages, &nr);
+ local_irq_restore(flags);
+ }

return nr;
}

-#ifndef gup_fast_permitted
-/*
- * Check if it's allowed to use __get_user_pages_fast() for the range, or
- * we need to fall back to the slow version:
- */
-bool gup_fast_permitted(unsigned long start, int nr_pages, int write)
-{
- unsigned long len, end;
-
- len = (unsigned long) nr_pages << PAGE_SHIFT;
- end = start + len;
- return end >= start;
-}
-#endif
-
/**
* get_user_pages_fast() - pin user pages in memory
* @start: starting user address
@@ -1710,12 +1719,22 @@ bool gup_fast_permitted(unsigned long start, int nr_pages, int write)
int get_user_pages_fast(unsigned long start, int nr_pages, int write,
struct page **pages)
{
+ unsigned long addr, len, end;
int nr = 0, ret = 0;

start &= PAGE_MASK;
+ addr = start;
+ len = (unsigned long) nr_pages << PAGE_SHIFT;
+ end = start + len;
+
+ if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
+ (void __user *)start, len)))
+ return 0;

if (gup_fast_permitted(start, nr_pages, write)) {
- nr = __get_user_pages_fast(start, nr_pages, write, pages);
+ local_irq_disable();
+ gup_pgd_range(addr, end, write, pages, &nr);
+ local_irq_enable();
ret = nr;
}

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